1. Field of the Invention
The present invention relates generally to a laser-diode-pumped solid-state laser which comprises a solid-state laser rod pumped by a semiconductor laser (laser diode), and more particularly to a laser-diode-pumped solid-state laser which includes an optical wavelength converter device disposed in a resonator for converting the wavelength of a laser beam which is oscillated by a solid-state laser rod.
2. Description of the Prior Art
U.S. Pat. No. 4,656,635, for example, shows a laser-diode-pumped solid-state laser in which a solid-state laser rod doped with a rare-earth material such as neodymium is pumped by a semiconductor laser. In order to obtain a laser beam having a shorter wavelength, the laser-diode-pumped solid-state laser includes a bulk monocrystal of nonlinear optical material disposed in a resonator for converting the wavelength of a laser beam which is oscillated by the solid-state laser into the wavelength of a second harmonic or the like.
As disclosed in the above publication, the nonlinear optical material used is an inorganic optical material such as KTP, LiNbO.sub.3, or the like. The inorganic optical material is however problematic in that the efficiency with which the wavelength is converted thereby is low, and hence the efficiency with which the energy is utilized is also low. If a highly intensive laser beam having a converted wavelength (i.e., a shorter wavelength) is desired, then an expensive semiconductor laser of a very high output power of such as 200 mW or more is required as the pumping source. If such a high-output-power semiconductor laser is employed, a large and expensive system for radiating the heat from and hence cooling the semiconductor laser is also needed, since a large amount of heat is radiated by the semiconductor laser.
The wavelength conversion efficiency may be increased by using a large crystal which provides a long path for the laser beam, as the bulk single crystal of nonlinear optical material. However, it is technically difficult and highly costly to produce such a large crystal.
An increased wavelength conversion efficiency may also be achieved by using a nonlinear optical material having a larger nonlinear optical constant. Inorganic optical materials having nonlinear optical constants which are larger than that of KTP include LiNbO.sub.3, BNNB, and KNbO.sub.3 which is disclosed in Optics Letters, Vol. 13, page 137 (1988), for example. These inorganic nonlinear optical materials, however, fail to provide a stable wavelength conversion efficiency over a wide temperature range because the phase matching angle of these materials tends to shift due to temperature change.
If the efficiency with which the solid-state laser is oscillated by the semiconductor laser is high, then the intensity of the oscillating laser beam that is applied to the nonlinear optical material becomes high, resulting in a wavelength conversion beam of a high intensity. However, the conventional laser-diode-pumped solid-state laser general employs an array laser as the pumping source. Since the spectral line width of the array laser is as large as 10 nm, the efficiency with which the solid-state laser is oscillated is low and the energy utilization efficiency is also low.
A single-transverse-mode, single-longitudinal-mode semiconductor laser as a semiconductor laser having a small spectral line width (which is normally as large as about 0.1 nm) is known. The oscillation efficiency of the solid-state laser can be increased by controlling the temperature of the single-transverse-mode, single-longitudinal-mode semiconductor laser with a Peltier device so that the oscillation wavelength of the laser will match the absorption peak value of the solid-state laser. However, the presently available single-transverse-mode, single-longitudinal-mode semiconductor laser has a lower output power than the array laser. In order to produce a wavelength conversion laser beam of a certain high intensity, the laser beams emitted by a plurality of single-transverse-mode, single-longitudinal-mode semiconductor lasers must be combined into a pumping laser beam. Such a system is costly to manufacture and low in reliability.